1. Field of the Invention
The present invention relates to a magnetic element which exhibits a rapid charge in magnetization with a change in an externally applied magnetic field, and a process for producing the same.
2. Description of the Related Art
There are many devices which utilize the magnetization behavior of a magnetic material. In addition to devices which exhibit a continuous response to a change in an external magnetic field such as a magnetic induction type magnetic head, magnetic elements made of a magnetic material which exhibit a sudden magnetic reversal and a discontinuous response when the intensity of the applied magnetic field exceeds a predetermined value have recently been employed. When a pickup coil is disposed in the vicinity of such a magnetic material, a steep voltage pulse can be produced in the coil upon a discontinuous magnetic reversal of the magnetic material. The use of such a magnetic element can provide a simplified apparatus which is widely applicable to the measurement of magnetic fields such as the earth's magnetic field, rotational speed, flow rate, etc.
Furthermore, in recent years, electronic article surveillance systems or identification systems for preventing the theft of merchandise or for rapidly processing the flow of materials have become more widely used. These devices employ identifying markers such as an oscillation circuit, an LC resonance circuit, a magnetostrictive vibrating material and a high magnetic permeability material, as well as the above-describe magnetic material which exhibits a discontinuous magnetic reversal. For example, U.S. Pat. Nos. 4,660,025, 4,686,516 and 4,797,658 disclose a system employing a marker made of a fine amorphous Fe based alloy wire. The magnetization of the foregoing fine metal wire material is extremely stable in the longitudinal direction and thus exhibits a very sudden 180.degree. magnetic reversal when the magnetic field reaches a predetermined magnitude. This characteristic is called a large Barkhausen discontinuity. When the intensity of an alternating magnetic field which has been transmitted as an inquiry signal in a monitor zone reaches a critical value, the fine metal wire exhibits a discontinuous magnetic reversal, thereby causing a detection coil to produce a steep pulse voltage. The waveform of the pulse voltage thus produced is then subjected to a frequency analysis in which the intensity and proportion of high harmonics are determined to identify the marker or to judge if it is necessary to sound an alarm. This system is advantageous in that the marker is inexpensive and provides an identifying capacity higher than that of other systems.
Magnetic materials have been found which exhibit a discontinuous magnetization response besides the foregoing fine amorphous metal wire. For example, U.S. Pat. Nos. 4,980,670 and 5,313,192 disclose a material obtained by subjecting a slender amorphous metal ribbon to heat treatment in a magnetic field. Furthermore, U.S. Pat. No. 5,181,020 discloses a thin film having a strong uniaxial magnetic anisotropy formed on a polymer substrate such as a plastic film which exhibits a discontinuous magnetic reversal. This material exhibits excellent square hysteresis characteristics similar to the fine metal wire.
The thin film disclosed in the above-cited U.S. Pat. No. 5,181,020 provides a steep discontinuous magnetization response similar to a fine amorphous metal wire when it is formed into a slender shape, e.g., 1 mm wide.times.50 mm long.times.0.5 .mu.m thick, along the direction of magnetic easy axis. However, these thin films are disadvantageous in that they are strongly affected by a demagnetizing field. Thus, these thin films exhibit remarkably deteriorated magnetic characteristics when they are formed into a shorter, wider and thicker shape. In recent years, there has been a great demand for miniaturized sensors or magnetic markers for preventing theft. However, these materials cannot adequately meet this demand.